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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_lexical_cast.frequently_asked_questions"></a><a class="link" href="frequently_asked_questions.html" title="Frequently Asked Questions">Frequently
    Asked Questions</a>
</h2></div></div></div>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Why does <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="identifier">int8_t</span><span class="special">&gt;(</span><span class="string">"127"</span><span class="special">)</span></code> throw <code class="computeroutput"><span class="identifier">bad_lexical_cast</span></code>?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> The type <code class="computeroutput"><span class="identifier">int8_t</span></code>
                is a <code class="computeroutput"><span class="keyword">typedef</span></code> to <code class="computeroutput"><span class="keyword">char</span></code> or <code class="computeroutput"><span class="keyword">signed</span>
                <span class="keyword">char</span></code>. Lexical conversion to
                these types is simply reading a byte from source but since the source
                has more than one byte, the exception is thrown. Please use other
                integer types such as <code class="computeroutput"><span class="keyword">int</span></code>
                or <code class="computeroutput"><span class="keyword">short</span> <span class="keyword">int</span></code>.
                If bounds checking is important, you can also call <a href="../../../libs/numeric/conversion/doc/html/boost_numericconversion/improved_numeric_cast__.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric_cast</span></code></a>: <code class="computeroutput"><span class="identifier">numeric_cast</span><span class="special">&lt;</span><span class="identifier">int8_t</span><span class="special">&gt;(</span><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="string">"127"</span><span class="special">));</span></code>
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Why does <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">&gt;(</span><span class="string">"127"</span><span class="special">)</span></code>
          throw <code class="computeroutput"><span class="identifier">bad_lexical_cast</span></code>?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> Lexical conversion to any
                char type is simply reading a byte from source. But since the source
                has more than one byte, the exception is thrown. Please use other
                integer types such as <code class="computeroutput"><span class="keyword">int</span></code>
                or <code class="computeroutput"><span class="keyword">short</span> <span class="keyword">int</span></code>.
                If bounds checking is important, you can also call <a href="../../../libs/numeric/conversion/doc/html/boost_numericconversion/improved_numeric_cast__.html" target="_top"><code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">numeric_cast</span></code></a>: <code class="computeroutput"><span class="identifier">numeric_cast</span><span class="special">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">&gt;(</span><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="string">"127"</span><span class="special">));</span></code>
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> What does <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;</span></code> of an <code class="computeroutput"><span class="identifier">int8_t</span></code>
          or <code class="computeroutput"><span class="identifier">uint8_t</span></code> not do what
          I expect?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> As above, note that int8_t
                and uint8_t are actually chars and are formatted as such. To avoid
                this, cast to an integer type first: <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;(</span><span class="keyword">static_cast</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">n</span><span class="special">));</span></code>
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> The implementation always resets
          the <code class="computeroutput"><span class="identifier">ios_base</span><span class="special">::</span><span class="identifier">skipws</span></code> flag of an underlying stream object.
          It breaks my <code class="computeroutput"><span class="keyword">operator</span><span class="special">&gt;&gt;</span></code>
          that works only in presence of this flag. Can you remove code that resets
          the flag?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> May be in a future version.
                There is no requirement in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1973.html" target="_top">Lexical
                Conversion Library Proposal for TR2, N1973 by Kevlin Henney and Beman
                Dawes</a> to reset the flag but remember that <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1973.html" target="_top">Lexical
                Conversion Library Proposal for TR2, N1973</a> is not yet accepted
                by the committee. By the way, it's a great opportunity to make your
                <code class="computeroutput"><span class="keyword">operator</span><span class="special">&gt;&gt;</span></code>
                more general. Read a good C++ book, study <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">sentry</span></code>
                and <a href="../../../libs/io/doc/ios_state.html" target="_top"><code class="computeroutput"><span class="identifier">ios_state_saver</span></code></a>.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Why <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">int</span><span class="special">&gt;(</span><span class="string">"-1"</span><span class="special">);</span></code>
          does not throw, but outputs 4294967295?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>
                has the behavior of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span></code>,
                which uses <code class="computeroutput"><span class="identifier">num_get</span></code>
                functions of <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">locale</span></code>
                to convert numbers. If we look at the Programming languages — C++,
                we'll see, that <code class="computeroutput"><span class="identifier">num_get</span></code>
                uses the rules of <code class="computeroutput"><span class="identifier">scanf</span></code>
                for conversions. And in the C99 standard for unsigned input value
                minus sign is optional, so if a negative number is read, no errors
                will arise and the result will be the two's complement.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Why <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">L</span><span class="char">'A'</span><span class="special">);</span></code> outputs
          65 and <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">wchar_t</span><span class="special">&gt;(</span><span class="identifier">L</span><span class="string">"65"</span><span class="special">);</span></code> does not throw?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> If you are using an old
                version of Visual Studio or compile code with /Zc:wchar_t- flag,
                <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code> sees single <code class="computeroutput"><span class="keyword">wchar_t</span></code> character as <code class="computeroutput"><span class="keyword">unsigned</span> <span class="keyword">short</span></code>.
                It is not a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>
                mistake, but a limitation of compiler options that you use.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> Why <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;(</span><span class="string">"-1.#IND"</span><span class="special">);</span></code> throws <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_lexical_cast</span></code>?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> <code class="computeroutput"><span class="string">"-1.#IND"</span></code>
                is a compiler extension, that violates standard. You shall input
                <code class="computeroutput"><span class="string">"-nan"</span></code>, <code class="computeroutput"><span class="string">"nan"</span></code>, <code class="computeroutput"><span class="string">"inf"</span></code>
                , <code class="computeroutput"><span class="string">"-inf"</span></code> (case
                insensitive) strings to get NaN and Inf values. <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span></code> outputs <code class="computeroutput"><span class="string">"-nan"</span></code>,
                <code class="computeroutput"><span class="string">"nan"</span></code>, <code class="computeroutput"><span class="string">"inf"</span></code>, <code class="computeroutput"><span class="string">"-inf"</span></code>
                strings, when has NaN or Inf input values.
              </li></ul></div>
        </li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
          <span class="bold"><strong>Question:</strong></span> What is the fastest way to convert
          a non zero terminated string or a substring using <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">lexical_cast</span></code>?
          <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
                <span class="bold"><strong>Answer:</strong></span> Use <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">iterator_range</span></code>
                for conversion or <code class="computeroutput"><span class="identifier">lexical_cast</span></code>
                overload with two parameters. For example, if you whant to convert
                to <code class="computeroutput"><span class="keyword">int</span></code> two characters
                from a string <code class="computeroutput"><span class="identifier">str</span></code>,
                you shall write <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">make_iterator_range</span><span class="special">(</span><span class="identifier">str</span><span class="special">.</span><span class="identifier">data</span><span class="special">(),</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">data</span><span class="special">()</span> <span class="special">+</span> <span class="number">2</span><span class="special">));</span></code>
                or <code class="computeroutput"><span class="identifier">lexical_cast</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;(</span><span class="identifier">str</span><span class="special">.</span><span class="identifier">data</span><span class="special">(),</span>
                <span class="number">2</span><span class="special">);</span></code>.
              </li></ul></div>
        </li></ul></div>
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<td align="right"><div class="copyright-footer">Copyright © 2000-2005 Kevlin Henney<br>Copyright © 2006-2010 Alexander Nasonov<br>Copyright © 2011-2021 Antony Polukhin<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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